Bone anchor screws come in a variety of shapes and sizes. One of the more common styles has a polyaxial head that allows for the screw to enter the bone structure at an ideal or preferred inclination. To achieve this polyaxial inclination, the head has a shape configured to rotate about its lower external surface. This lower surface can be one of a number of shapes like conical or spherical or hemispherical. This ability is often used in devices having a modular head assembly.
The bone screw assembly generally includes a tulip. A tulip is a body structure having two opposing sides spaced by a slotted opening to receive a spinal rod. The tulip often employs internal threads to receive a rod locking set screw to anchor or fix the rod in the tulip. The lower portion of the tulip has an opening to receive the bone screw in a base seat. Often, the tulip can have a saddle that both supports the rod along an underside of the rod. The saddle having an upper recessed curvature into which the rod sits and a lower cup like opening to receive the top of the bone screw head. When the saddle and rod and set screw are tightened, the screw angle is fixed against the tulip seat.
Often, it is preferred that the bone screw is first placed securely in the bone structure leaving the head protruding above the bone surface. In this surgical procedure the tulip assembly must be adapted to fit down onto the projecting screw head. To accomplish this, the surgeon must push the tulip onto and over the screw head without a clear path of vision. Accordingly, the placement must be accomplished without any way of knowing if the tulip or other device is properly secured. Thereafter, the device is tightened to complete the assembly and the only way to insure the assembly is secure requires an upward pulling of the tightened assembly. This works well when fixation of two adjacent vertebrae is being performed and the length of the fixation rod is relatively short.
If, however, the span is long, as it is when three vertebrae segments are being fixed, this often requires the use of a bent rod or requires the surgeon to bend the rod to custom fit the patient.
The present invention solves this problem in a unique one piece structure that allows a second rod to be used in a tulip that is particularly crafted to accommodate two rods while employing a single bone screw in the assembly.